My invention is in the field orthopedic surgical guides and jigs. Specifically, I have invented an medullary caliper for use in connection with the implantation of prosthetic joints.
In the field of orthopedic prostheses, diseased or injured natural joints are replaced with prosthetic devices. Joints in the hip, knee, shoulder, elbow, ankle and fingers have been replaced with prostheses. Such prostheses are secured to adjacent bone by a number of known means. Such means include bone screws, pegs or other securing devices which may be used alone or in combination. One of the means frequently employed is a shaft or stem which is extends into the medullary canal of the adjacent bone. This is the method most frequently used in replacing a defective head for a femur in a hip prosthesis, but the means has also been used in prosthetic knees, fingers and shoulders. To implant such a prosthesis, a portion of the joint is cut away, exposing a resected surface of the bone. A cavity is reamed into the bone to receive the prosthesis and the prosthesis is placed into the reamed cavity with the stem extending into the medullary canal. The prosthesis is secured against lateral movement by a combination of features such as bone cement, distal spacers on the end of the stem of the prosthesis, or auxiliary sleeves placed on the stem of the prosthesis. Distal spacers are known, for example from Lee, U.S. Pat. No. 4,753,657 or Ling, U.S. Pat. No. 3,793,650. Sleeves have been used in the so-called Whiteside Hip produced by Dow Corning Wright. Another type distal size means has been disclosed by Averill in U.S. Pat. No. 4,770,660. Each of these distal stem features provide means for altering the size of a prosthesis to meet the particular physiological requirements of a patient. The effective use of such features depends on the fit achieved between the distal feature of the prosthetic stem and the medullary canal. It is, therefore, important for a surgeon to know, as accurately as possible, the diametric dimension of the medullary canal at the location where the distal feature will be employed.
In the past, surgeons have relied on their general experience or have attempted to estimate the size of an medullary canal from x-ray photographs. Neither of these methods is reliably accurate. X-ray photography, for example, does not provide a fixed scale nor is it immediately clear that all features in such a photograph reside in any given plane. It is, therefore, very difficult to get an accurate measurement of absolute size from a an x-ray photograph.
It is an object of my invention, therefore, to provide an apparatus for measuring diametrical dimensions of an medullary canal.
It is a further object of my invention, to provide such an apparatus which can give accurate multiple readings at selected depths within the medullary canal.
Another object of my invention is to provide an apparatus which can provide Medullary measurements in multiple selected directions.
A further object of my invention is to provide a caliper which can be used by a surgeon during a surgical procedure to accurately size distal features of a prosthesis.